Selection of procedures in mental subtraction.

Adults solved simple subtraction problems (e.g., 16 - 9). Half of the 32 participants provided immediate self-reports of their solution processes on each problem. Performance was analyzed using both traditional descriptive statistics (i.e., means, standard deviations, and percentage of errors) and with statistics derived from fitting the ex-Gaussian distributions to latencies (i.e., mu and tau). The results support the view that ex-Gaussian analyses can be useful in exploring patterns of procedure selection that relate both to characteristics of the stimuli (e.g., problem size) and to characteristics of the participants (e.g., arithmetic skill). More generally, the results provide further evidence that adults use a variety of procedures to solve simple subtraction problems and that these choices are related to patterns of performance on more complex problems that require calculation.